1. Field of the Invention
The present invention relates to an optical sheet suitable for a light diffusing sheet or the like built in a back light unit of a liquid crystal display, for example, and a back light unit using the optical sheet.
2. Description of the Related Art
For liquid crystal displays, a back light system in which a back surface of a liquid crystal layer is illuminated to thereby emit light has spread, and the back light unit is provided on a lower surface side of the liquid crystal layer. In general, as shown in FIG. 6, a back light unit 20 comprises a bar-shaped lamp 21 as a light source and a plurality of laminated optical sheets 22. Each of the optical sheets 22 has a particular optical characteristic such as refraction and diffusion. Specifically, these optical sheets 22 comprise a light guiding plate 23 whose ends are adapted to those of the lamp 21, a light diffusing sheet 24 provided on a front-surface side of the light guiding plate 23, and a prism sheet 25 provided on a front-surface side of the light diffusing sheet 24.
A function of the back light unit 20 will now be described. Light rays emitted from the lamp 21 enter the light guiding plate 23, are reflected by reflection dots or a reflection sheet (not shown) provided on a back surface of the light guiding plate 23 and side surfaces thereof, and emanate from a surface thereof. Then, the light rays emanating from the light guiding plate 23 enter the light diffusing sheet 24, are diffused therein, and emanate from a surface thereof. Then, the light rays emanating from the light diffusing sheet 24 enter the prism sheet 25, and emanate from prism portions 25a formed on a surface thereof as light rays exhibiting distribution having its peak in a substantially vertical direction. Thus, the light rays emitted from the lamp 21 are diffused by the light diffusing sheet 24 and refracted by the prism sheet 25 into the light rays having its peak in the substantially vertical direction, thereby illuminating the entire surface of the liquid crystal layer (not shown) provided above.
There is also a back light unit further provided with a light diffusing sheet or a prism sheet as the optical sheets 22 taking a light gathering characteristic of the prism sheet 25 into account, although this is not shown.
Since the light rays enter the light guiding plate 23 from its end side on which the lamp 21 is provided, the light rays cannot emanate from the light guiding plate 23 in the vertical direction thereof even if a reflection mechanism of the back surface thereof is devised, and a peak direction of the light rays emanating from the surface of the light guiding plate 23 is inclined at a predetermined angle toward an opposite side of the lamp 21. The direction of such inclination is determined depending on the type of the light guiding plate 23. Also, as for the prism sheet 25, the peak direction of the incident (incoming) light rays that is suitable for refracting the peak direction of the emanating light rays to vertical is determined by its shape.
Accordingly, it is desirable that the light diffusing sheet 24 be capable of guiding the predetermined peak direction of the light rays emanating from the light guiding plate 23 to the peak direction suitable for the incident light rays of the prism sheet 25.
By way of example, the conventional light diffusing sheet 24 is a light diffusing layer in which a light diffusing material such as resinous beads is dispersed on a surface thereof, or a light diffusing sheet having fine convex and concave regions on a surface thereof by embossing and the like. The main function of these light diffusing sheets 24 is to diffuse the penetrating light rays, and an angle changing ability to change the peak direction of the emanating light rays to vertical is added by the light diffusing ability. For this reason, in order to increase the angle changing ability of the conventional light diffusing sheet 24 to direct the penetrating light rays to a normal line direction, it is necessary to increase the light diffusing ability. In actuality, however, if the light diffusing ability is greatly increased, then the required amount of light rays emanating in the normal line direction is reduced and luminance of the liquid crystal display using the back light unit 20 is correspondingly reduced.
The present invention is directed to solving the above-described problem, and it is an object of the present invention to provide an optical sheet capable of directing a peak of passing light rays to vertical without increasing a light diffusing ability and a back light unit using the optical sheet.
In one aspect of the present invention, there is provided an optical sheet comprising a base layer made of transparent synthetic resins and having an optical characteristic, wherein the base layer is formed of different materials alternately arranged. With this optical sheet, it is possible to direct the light rays penetrating the base layer to a normal line direction by reflection or refraction at an interface between the different materials alternately arranged. Therefore, it is not necessary to increase the light diffusing ability to increase the angle changing ability unlike the conventional light diffusing sheet. Consequently, it is possible to increase the angle changing ability without reducing the amount of light rays emanating in the normal line direction.
Preferably, materials of different refractive indices are used as the different materials. The alternate arrangement of the materials of the different refractive indices effectively brings about reflection or refraction at the interface between the different materials alternately arranged.
When the materials of the different refractive indices are used as the different materials as described above, the refractive index may be continuously changed at the interface between the materials alternately arranged. Such changing of the refractive index at the interface between the different materials alternately arranged provides an effect in which each of the synthetic resins alternately arranged is a grated-type fiber that enables to curve, reflect, and refract the light ray in the vicinity of the interface. Consequently, it is possible to direct the emanating light rays toward the normal line direction.
Preferably, the interface between the different materials alternately arranged is perpendicular to the surface of the sheet. Thereby, the optical sheet can be easily fabricated, and the movement of the light rays passing according to the refractive indices of the different materials, namely, the angle changing ability can be easily predicted and designed. On the other hand, the interface between the different materials may be inclined in a predetermined direction with respect to the surface of the sheet. Thereby, the light rays passing the interface can be refracted upwardly.
Preferably, the different materials are arranged in stripes. The arrangement of the different materials in stripes to form the base layer provides the angle changing ability only in a stripe direction and in a vertical direction.
On the other hand, since the peak direction of the light rays emanating from the light guiding plate has the light outgoing characteristic in which the peak direction of the light ray emanating from the light guiding plate is inclined at a predetermined angle toward the opposite side of the lamp as mentioned previously, this can be adapted to the characteristic of the light emanating from the light guiding plate by the angle changing ability of the optical sheet only in the stripe direction and in the vertical direction by using this optical sheet, thereby directing the light rays to the normal line direction.
Preferably, the arrangement width of each of the different materials is larger than the wavelength of the visible light ray. Thereby, the above-described angle changing ability is effectively exhibited. Conversely, the arrangement width may be set to not larger than a wavelength of the visible light ray. In this case, the light rays passing the base layer are dispersed by diffraction. Therefore, the light diffusing ability can be greatly increased.
The optical sheet of the present invention is also applicable to a light diffusing sheet comprising a further light diffusing layer provided on a surface of the base layer. As the light diffusing layer, a binder in which resinous beads are dispersed may be used. In the optical sheet, the base layer has the angle changing ability and the light diffusing layer provided on the surface of the base layer has the light diffusing ability. For this reason, it is not necessary to increase the light diffusing ability for a larger angle changing ability unlike the conventional light diffusing sheet, and it is possible to add an angle changing ability as an ability separate from the light diffusing ability. Therefore, the amount of light emanating in the normal line direction is not reduced by the light diffusing ability if the angle changing ability is increased.
Preferably, the optical sheet of the present invention further comprises a sticking-proof layer provided on a rear surface of the base layer, in which resinous beads are dispersed in the binder. The provision of the sticking-proof layer on the rear surface of the base layer prevents sticking when the optical sheet is superimposed on another sheet.
Accordingly, with the use of the optical sheet of the present invention as the optical sheet in the back light unit for a liquid crystal display comprising a lamp and an optical sheet for uniformly guiding light rays emitted from the lamp to a front-surface side, the number of optical sheets required for directing the light rays to vertical can be reduced and the thinning of the back light unit is facilitated because of the higher angle changing ability described above. Besides, with the use of the optical sheet as the light diffusing sheet provided between the light guiding plate and the prism sheet, the optical sheet of the present invention has the angle changing ability that can be controlled relatively precisely and easily, and therefore can be adapted to the light outgoing characteristics of the light guiding plate and the prism sheet. Furthermore, it is not necessary to change the light diffusing ability for adjustment of the angle changing ability unlike the conventional light diffusing sheet, the luminance of the liquid crystal display can be improved without reducing the amount of light emanating in the normal line direction if the angle changing ability is increased.